Which algorithm is used in the robot Sophia to understand and answers the questions?
Sophia ,first , has all the questions and corresponding answers preprogrammed. It is a system which is a hybrid of Bayes Text classification and decision trees. 


It may consist of a speak recognizer which converts the question into a string.
This string then travels into a algorithm which gets a suitable answer for it.
The answer is then spoken by the speech synthesizer along with some face actions.
It might also have a sentiment analyser which identifies sentiments in a answer to show face expressions accordingly.
Along with this, it identifies the emotions on the face of the person sitting in front of it, so as to provide a greater expression.


Hence, Sophia understands or thinks nothing like the human brain. It is just a system which gives predefined answers corresponding to the question asked.